Effective recruitment and retention are essential to successful clinical research but we have little good evidence about how to achieve this. Joy Adamson and colleagues call for more use of methodological trials embedded within clinical trials to improve our knowledge

Randomised controlled trials form the bedrock of evidenced based clinical decision making. Many clinicians will be involved in recruiting, treating, and following up trial participants during their career. Although there are many randomised trials aiming to reduce treatment uncertainty, there are few trials of interventions to reduce uncertainty about how best to recruit and retain participants. For instance, is it better to use doctors or nurses to recruit? Only one trial—among patients with prostate cancer—has attempted to answer this question.[1]

Recruitment to and retention in trials are extremely important. Both problems increase the risk of a type II error—that is, incorrectly concluding that there is no clinically important difference between treatments. Attrition, arguably, is more worrying because as well as reducing the power of the study it potentially introduces bias, particularly if attrition differs between the randomised groups.[2] [3]

The lack of evidence makes it difficult to select the most appropriate recruitment and retention strategies when designing clinical trials, contributing to the waste of research resources.[4] Indeed, most clinical trials do not recruit on time and to target, which delays the acquisition of clinical evidence as well as adding to costs.[2] Improving the evidence therefore needs to be a priority.

Problems with the evidence

Although there are many evaluations of interventions to improve recruitment and retention, few of these use randomisation. Two recent Cochrane reviews lament the paucity of evidence to inform trialists and clinicians on the best strategies.[5] [6] Treweek and colleagues found 27 randomised trials evaluating recruitment strategies, only 19 of which were in the context of a real as opposed to …